Watching more mature stuff with kids as they grow older

So I'm 30+ looking for advice. Mainly with one kid in particular, but I have many kids in my family I watch stuff with. We're used to watching shonen anime, so a few perverted characters is nothing new, but those shows also have the perverts getting smacked around or reprimanded for their deeds.

My nephew wanted to watch Assassination Classroom the other day. I read the parent guide on it, and it seems to have lewd scenes for the same of lewdness. For example one note reads:

>A female character asks a male character how she looks in a tube top that exposes a lot of cleavage. The close up on her chest lasts a couple seconds. The male character is very pleased, though the female character doesn't understand why.

I don't really know how to talk about something like this with my nephew and nieces who are getting to an age where they're going to be watching more than by stuff. Talking about sex clinically is pretty easy. Talking about desire and such is a whole different story.